A great deal of research has been directed over the last several decades toward characterizing the composition and properties of carbon nanomaterials, and developing efficient pathways for making, purifying and using this unique class of materials. The extraordinary mechanical, electronic and chemical properties of carbon nanotubes, for example, have stimulated significant interest in developing applied technologies exploiting these useful properties. As a result, carbon nanotubes are currently identified as a promising class of functional materials for electronic and opto-electronic systems, and there is substantial interest in developing advanced techniques for integrating carbon nanotube materials into useful devices and systems.
Carbon nanotubes are allotropes of carbon comprising one or more cylindrically configured graphene sheets, which are classified on the basis of structure as either single walled carbon nanotubes (SWNTs) or multiwalled carbon nanotubes (MWNTs). Having small diameters (≈1-30 nanometers) and large lengths (up to several microns), SWNTs and MWNTs are characterized by very large aspect ratios (i.e., length to diameter ratio ≈103 to about 105). Carbon nanotubes exhibit an energy band structure that varies considerably depending on their precise molecular geometry and chemical environment, and are either metallic or semiconducting in electronic character. Charge transfer doping and intercalation reactions have been shown to provide effective techniques for accessing doped carbon nanotube materials having selected electronic and chemical properties.
Single walled carbon nanotubes (SWNTs), in particular, are identified as functional materials potentially enabling a new generation of high performance passive and active nanotube-based electronic and opto-electronic devices. SWNTs comprise a single, contiguous graphene sheet wrapped around and joined with itself to form a hollow, seamless tube having capped ends. Typically having extremely small cross sectional dimensions (e.g., diameters ≈1-5 nanometer), SWNTs are often present in bundled configurations. SWNTs have been demonstrated to exhibit a high degree of chemical versatility and are capable of functionalization of their exterior surfaces and encapsulation of a range of materials within their central hollow cores, including gases, molten materials and biological materials.
SWNTs exhibit attractive electrical properties for target applications in micro- and macroelectronics. The electron transport behavior in carbon nanotubes, for example, is predicted to be essentially that of a quantum wire, which has stimulated interest in fabricating ultrafast nanotube based devices. In addition, charge transfer and intercalation doping processing pathways provide avenues for selectively tuning the electrical and chemical properties of carbon nanotubes to facilitate integration in specific electronics applications. SWNTs have also been demonstrated to have very high intrinsic field effect mobilities (e.g., up to about 9000 cm2V−1s−1) making them interesting for possible applications in nanoelectronics. Due to their nanometer size diameter, mechanical robustness, chemical stability and high electrical conductivity, SWNTs are being developed as enhanced field emitters in a range of devices, including flat panel displays, AFM tips and electron microscopes. SWNTs also possess useful thermal, mechanical, magnetic and optical properties which make them suitable materials for other nanotube-based emerging technologies. The unique mechanical properties of SWNTS, for example, make them attractive as additives in high strength, low weight and high performance composite materials. Calculations and experimental results suggest that SWNTs have tensile strengths at least 100 times that of steel or any known other known fiber, and that SWNTs are stiffer than conventional reinforcement materials, such as carbon fibers, while also exhibiting a very large Young's Modulus (as large as about 1 TPa) when distorted in some directions.
The unique combination of electronic and mechanical properties of SWNTs, make them particularly well-suited for use in large-scale distributed electronics. Target applications currently of high interest include high resolution display systems (e.g., flat panel displays, cathode ray tubes, liquid crystal displays, etc.), steerable antenna arrays, flexible electronics, and large area sensors. High resolution patterning of carbon nanotubes is a necessary processing step for many of these applications. Commercial implementation of this technology, therefore, requires the development of advanced techniques for patterning, assembling and integrating carbon nanotubes capable of satisfying important device processing requirements. First, processing techniques must be capable of patterning in a manner which avoids degradation or damage of the nanotubes, thereby retaining the electronic and mechanical properties necessary to supporting implementation in high performance electronic devices. Second, nanotube patterning techniques must be sufficiently versatile to support device fabrication on a range of electronic device substrates, including flexible, polymer-based substrates and substrates prepatterned with device components. Third, processing techniques must be capable of commercially feasible implementation, for example, for high resolution patterning and/or large area patterning applications.
Given their high degree of chemical and physical stability, conventional patterning techniques are inadequate to fully support commercial implementation of nanotube-based electronic systems. As a result, a number of new processing strategies have been proposed and explored to address the present need for nanotube-based electronic devices. Despite this activity, there is currently a significant need for the development of processing techniques for patterning carbon nanotubes.
Choi et al. report carbon nanotube patterning using an electrophoresis method, wherein the nanotubes are grown selectively on a patterned indium tin oxide (ITO) layer. [See, W. B. Choi et al., Electrophoresis deposition of carbon nanotubes for triode-type field emission display, Applied Physics Letters, vol. 78, no. 11, p 1547, 2001]. The authors report that the technique is capable of generating a high resolution pattern of the electrophoresis deposited carbon nantoubes on the ITO underlayer. The processing strategy described by Choi et al. is susceptible to significant drawbacks, however, that hinder its commercial implementation. The patterning technique requires a resource intensive prepatterning step of the ITO underlayer to generate the desired carbon nanotube pattern. Also, it is expected that the physical and electronic characteristics of carbon nanotubes deposited by electrophoresis may not be sufficient to support applications in high performance electronics, as it is generally accepted that carbon nanotubes grown by high temperature chemical vapor deposition (CVD) techniques have superior properties. Choi et al. also describe a screen printing-based processing technique for patterning carbon nanotube materials. [See, W. B. Choi et al., Fully sealed, high-brightness carbon-nanotube field-emission display, Applied Physics Letters, vol. 75, no. 20, p 3130, 1999]. In these methods, solvents are added to a carbon nanotube-containing powder to make paste that is subsequently printed selectively on a substrate for a field emission display device. While the method is reported to provide a pattern having reasonable fidelity on the device substrate, the deposited carbon nanotubes are also likely to have inferior electronic and physical properties relative to methods using CVD to generate nanotubes directly on the substrate. The screen-printing approach is also likely to be susceptible to problems arising from low pattern resolution and inefficient use of the carbon nanotube material.
Oh et al. report room temperature methods using self-assembly of carbon nanotubes to achieve patterning on substrates. [See, S. Oh et al., Room temperature fabrication of high resolution carbon nanotube field emission cathodes by self-assembly, Applied Physics Letters, vol. 82, no. 15, 2521, 20]. In the reported method, alternating hydrophilic and hydrophobic regions are defined on the substrate surface prior to carbon nanotube deposition. A homogeneous suspension of carbon nanotubes is then applied to the substrate surface having the hydrophilic and hydrophobic regions. Carbon nanotubes are demonstrated to only coat the hydrophilic surface, thereby achieving substrate patterning. Similar to the work of Choi et al., although patterning was achieved, the deposited carbon nanotubes are likely to have inferior electronic and physical properties relative to methods using CVD to generate nanotubes directly on the substrate undergoing processing.
S. Lu et al. disclose a patterning process reportedly capable of generating high resolution patterns exhibiting fine features of patterned carbon nanotubes. [S. Lu et al., Nanotube micro-optomechanical actuators, Applied Physics Letters, vol 88, p 253107, 2006]. In the disclosed technique, a uniform layer of carbon nanotubes of a desired thickness is first generated on a substrate surface by compressive loading. The deposited layer of carbon nantoubes is subsequently pattered by a combination of processing steps involving selective masking using conventional photolithography followed by selective removal of carbon nantoubes in exposed regions using an oxygen plasma. It is expected that the oxygen plasma attacks the defects in the carbon nanotubes, and the etching of carbon nanotubes progresses from the defect point to the normal structure. A disadvantage of this technique is that selective removal of the carbon nanotubes using oxygen plasma requires long processing times, which hinders commercial implementation of this technique.
Other references that disclose carbon nanotube processing techniques for making electronic devices include U.S. Pat. Nos. 7,264,990, 6,988,925, 6,277,318 and 6,348,295 and International Patent Publication No. WO 2005/086982.